Dan Page wrote:
> Probably a real simple question but wonder if someone can straighten me
out.
> I got some data in Arcview SHP files in which they told me the projection
is a
> Geographic Projection or Lat/Long (No projection).
> Now my question is even if the data is in Lat/Long geographic coordinates
does
> it not need to be associated with a datum like Nad83 or something else?
> If lat/long coordinates can have no datum associated do I import it in
MapInfo
> using the first Lat/Long projection system listed (which doesn't seem to
> indicate a datum)? I also have a lot of MapInfo data that is in this first
> lat/long (no datum indicated) projection system, does that mean all this
data
> has no datum? I've heard that if data is in this first Lat/Long projection
> system listed in MapInfo and then later decide to do a "save copy as" to
convert
> it to Nad83 for example, that MapInfo doesn't convert it to Nad83 it just
> associates your data as being Nad83 now and no coordinates have been
converted,
> is that true? Can anyone clear this up for me.
A projection with no datum can mean either "Treat the earth as a sphere" or
"I don't know which datum". There is no way in MapInfo to separate the
two concepts. It may not matter.
In your case, don't add information you don't know about. If you don't know
which
datum a map table is associated with, leave it associated with "no datum".
You are right: "File -> Save Copy As" will not convert coordinates when you
saving a "lat/long, no datum" table to a "lat/long, some datum" table.
That's the
only time that will happen, however.
The only time you might want to assign a datum to such a table is when you
are going to make a map using map tables with layers using at least TWO
OTHER datums (data?). Because of the unaviodable inaccuracy of the
"on-the-fly" datum conversion
algorithm, the different layers' graphic objects may not line up correctly.
In that
case, save a special copy of the table in one of the datums being used, and
use that copy
for that map only.
HTH
Spencer
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